Hammer mills have long been used for grinding or comminution of various materials. A typical hammer mill comprises a rotor assembly mounted on a rotor shaft inside a housing. A rotor assembly 1100 is illustrated at rest in FIGS. 11 and 13 of Plumb et al. U.S. Pat. No. 8,104,177. A material inlet is generally located at the top of the housing with one or more material outlets located near the bottom of the housing. As shown in FIGS. 11 to 13 of the Plumb et al. '177 Patent, the rotor assembly 1100 includes a drive shaft and rows of hammers 1400, as illustrated in FIG. 14 of the Plumb et al. '177 Patent. The hammers 1400 are pivotally connected to the rotor 1100 by a of a steel hammer rod or pin. The hammers are normally flat steel blades or bars, as illustrated in FIGS. 11 to 14. The hammers extend out substantially radially from the hammer rods due to inertia when the hammer mill is (rotating) in operation, as illustrated in FIG. 12. The rotor assembly 1100 is mounted inside a housing, known by those skilled in the hammer art as a grinding or working chamber. In a reversible hammer mill, this grinding chamber comprises a cutting plate mounted on either side of the material inlet.
Present-day cutting plates comprise an upper, linear section, and do not allow particles to escape. Downstream of the cutting plate, the interior of the working chamber is defined by curved screen plates. The screen opening diameter is selected to match the desired final particle size of the material being comminuted. Particles less than or equal to the desired size exit the chamber though the screens, while material greater than the desired size are further reduced by the rotating hammers.
Standard hammers, when grinding or comminuting materials, impact the product to be pulverized to create a smaller size particle. This impact forces material against a perforated screen area and cuts and sizes the product. Inside the typical hammer mill, numerous forces act. Forces exist at the contact end of the hammer, where the hammer impacts the material being comminuted and that is why many prior art hammers have hard facing welded to the contact end for reasons such as reducing wear on the contact end. Sometimes adding hard facing makes the grinding end of the hammer too large, causing the hammer mill to use more than an optimum amount of horse power to run it.
The Plumb et al. U.S. Pat. No. 8,104,177 also show a providing hard facing such as hardened material 22 added to edge portions of the contact end of the hammer, preferably by welding, to increase hammer life. See also Newell, U.S. Pat. No. 3,482,789 (lines 12 to 18 of column 4); Kachik, U.S. Pat. No. 4,856,170 (lines 55 to 68 of column 8); and, Lowry U.S. Pat. No. 4,129,262. Welding of tungsten carbide onto the contact surfaces of hammers is well known in the art, as illustrated in the Young U.S. Pat. No. 7,140,569 (lines 9 to 17 of column 3); Young, U.S. Pat. No. 8,033,490 (lines 28 to 36 of column 9); Balvanz, U.S. Pat. No. 6,419,173 (lines 11 to 16 of column 3); Rogers, U.S. Pat. No. 2,647,695 (lines 37 to 41 of column 2); Mankoff, U.S. Pat. No. 2,763,439 (lines 26 to 31 of column 1); and, Eilers, U.S. Pat. No. 3,045,934 (lines 44 to 54 of column 2). Hammers produced by Jacobs Corporation, as illustrated in Ronfeldt et al., U.S. Pat. No. 7,419,109, included hardened material, such as tungsten carbide, welded on the contact edges of hammer mill hammers. It happens sometimes that hard facing separates from the hammer, which at best will only cause increased wear on the hammer and at worst will allow a loose piece of hard facing to be present in the material being comminuted.
Some prior art patents show grooves or notches in the grinding end of the hammer. See, for example, Williams, U.S. Pat. No. 1,760,097; Iglehart, U.S. Pat. No. 1,827,986; Alfred, U.S. Pat. No. 1,829,325; and, Jensen, U.S. Pat. No. 1,954,175. Tankersley, U.S. Pat. No. 2,237,510, shows grooves in the sides of a hammer extending from the contact end toward the connection end. All of the prior art patents referred to in this document are hereby incorporated by reference herein in their entirety.